The invention relates to a control arrangement for a multilevel convertor having a plurality of energy storage elements and a plurality of switching means, in particular, but not exclusively, such a multilevel convertor for connection to an AC system as a static var compensator (SVC).
Static var compensators are used in ratings typically from 1 MVar to 100 MVar or more in AC power transmission and distribution systems to control and stabilize AC voltage. These are devices normally connected in shunt to the AC system and can generate variable lagging or leading reactive current (or reactive volt-amperes--Vars) in dependence on a control system. Since the effective source impedance of an AC power system is almost always inductive, the AC voltage can thereby be changed, or alternatively it can, for example, be held constant in the presence of varying AC system load currents, by appropriate control of SVC current.
FIG. 1 shows an example of a typical arrangement as an SVC in elementary form for a single-phase system in which the AC source is represented as an equivalent of emf 1 behind a reactance 2 supplying a busbar 3. An attenuated version 9 of the voltage at busbar 3 is fed via a voltage transformer 8 to the control system 20. A load 4 is shown, and this may draw a varying current from busbar 3 such that the voltage of the latter varies. A power convertor 5 is shown connected in shunt to the busbar via a reactance 6. The control system 20 can, by suitable adjustment of the gating signals 10, change the convertor reactive current to effectively hold the voltage magnitude on busbar 3 constant, for example.
Alternative system arrangements and control functions other than constant voltage can also be implemented. For example, an SVC connected in shunt to the centre of a long transmission line connecting two AC generating systems can be controlled to change AC voltage, and consequently transmitted power, in such a manner as to dampen electromechanical oscillations and thereby stabilise the overall system.
Practical forms of SVC behave effectively as variable inductive or capacitive reactance, drawing variable reactive current but practically zero real current (neglecting SVC component losses). One well-known form of such a device is the thyristor-controlled reactor (TCR). Another is the force-commutated (voltage-source or current-source) convertor using electronic switching devices with turn-off capability; for high ratings these are normally gate-turn-off thyristors (GTOs). The SVC is then referred to as a GTO SVC.
The SVCs described above can be considered to comprise a variable reactive impedance connected in shunt to the AC system. It is also known to connect such a variable reactive impedance in series with a transmission line in an AC system, usually to interconnect two parts of the AC system, so as to improve system stability by control of AC system voltages. The general arrangement of this is shown in FIG. 2, in which one AC system is represented by emf 1 and impedance 2, the other similarly by emf 13 and impedance 12. The variable series impedance is shown as block 11.
The power converter block 5 is expanded in FIG. 3 to show possible power circuit connections to create a "multilevel circuit" using "H" bridges with which the control system of the present invention can be used. FIG. 3a shows the arrangement for a voltage-source convertor and FIG. 3b for a current-source convertor. Such a voltage-source convertor is the subject of the applicants'co-pending patent application GB 9422263.5. Multilevel circuits need a number of energy storage elements 14, which for the voltage-source convertor may be constituted by a capacitor or a battery, and for the current-source convertor may take the form of an inductor. Other more complex forms of energy storage element are also possible.
For the particular voltage-source convertor shown in FIG. 3a, each "H" bridge 15 has two AC terminals 16 and two DC terminals 17. Connected across the DC terminals 17 is, for example, a capacitor, and the three capacitors represented by the elements 14 can be connected in series in an appropriate switching pattern via switching elements 18, which may take the form of GTOs, to produce at the outer AC terminals 7, 35 of the convertor an AC waveform having a number of voltage levels equal to 2.times.(number of capacitors)+1, in this case 7. The waveform is therefore a stepped waveform which in most cases will be an approximation to a sine wave. Clearly, the greater the number of capacitors, the closer the sinusoidal approximation. The AC terminal 7 is connected to the AC system busbar via the inductor 6 (see FIG. 1).
The same principle applies to the current-source convertor shown in FIG. 3b, except that in this case the DC terminals of the "H" bridges are interconnected by inductors 13 and the bridges are connected not in series, but in parallel in the required switching sequence. This time the multilevel sinusoid is a current waveform which leads or lags the voltage on the busbar 3 by 90 electrical degrees.
A further example of a multilevel convertor which is suitable for control by the present invention is that which forms part of the subject-matter of an earlier co-pending patent application of the applicant, namely GB 9400285.4, which was published on Jul. 12, 1995.
The operation of a multilevel voltage-source convertor as an SVC is described with reference to FIG. 4. The convertor produces an almost sinusoidal voltage-source, V.sub.C, made up of a number of nominally equal DC voltage levels and is approximately in phase with the AC system voltage, V.sub.S. The coupling impedance L is usually the leakage reactance of a step-down transformer. By controlling the voltage V.sub.L across this inductance the SVC can vary the reactive current flow.
If V.sub.C and V.sub.S are in phase with each other, only reactive current can flow, as shown in the vector diagrams (a) and (b). In this case the magnitude of I.sub.L is proportional to the voltage difference V.sub.L between V.sub.C and V.sub.S. If V.sub.L is zero, there is no current flow. The sign of V.sub.L determines whether I.sub.L is leading (capacitive), as in FIG. 4(a) or lagging (inductive), as in FIG. 4(b).
If a phase shift is introduced between V.sub.C and V.sub.S, a component of real current will flow as shown in the vector diagrams (c) and (d). In this case the magnitudes of V.sub.C and V.sub.S are equal but there is a phase shift between them so only a component of real current flows. The magnitude of I.sub.L is proportional to the phase difference between V.sub.C and V.sub.S and the sign of this phase difference determines whether real current flows out of (FIG. 4(c)) or into (FIG. 4(d)) the convertor.
The capability to transiently exchange real current and hence power between the convertor and the AC system by introducing a phase shift between V.sub.C and V.sub.S is one means of controlling the level of SVC reactive current output. This is possible because, as the convertor cannot generate or absorb real current without the subsequent discharging or charging (respectively) of the capacitive storage elements, any real current flow will cause a change in the capacitor voltage levels and consequently a change in the reactive current output. Vector diagram (e) shows the case where there is both real and reactive power flowing through the convertor--in this case, the reactive power is inductive and real power flow is out of the convertor into the system.
Various methods of controlling the operation of switching devices are known, one of which is the zero-crossing detector. This was an early technique which related the instants at which the devices were fired to measurements with respect to the voltage zero crossings of the AC system to which the SVC was connected. An example is shown in FIG. 5, where .alpha. is the firing angle of a thyristor-controlled reactor (TCR). The reference waveform could be the actual measured voltage or the output of a phase-locked loop directly locked to the zero crossings of the measured voltage.
Another technique which was developed is Ainsworth's classic design of the phase-locked oscillator control system, illustrated in FIG. 6. FIG. 6 shows how this system is utilized for the control of a TCR. This is an indirectly phase-locked oscillator which generates firing instants from a half-cycle integral of the measured voltage, not from zero crossings, and is very stable. FIG. 7 shows the waveforms associated with this control system. Changes in firing instants occur if the integral of the error signal (the shaded areas in FIG. 7) changes.
Drawbacks are associated with these known control methods. In the first case, if the AC system to which the SVC is connected is a so-called weak system, i.e. one having a high series system inductance, the SVC could actually influence the zero crossings of the system itself with resultant instability of control. A problem with the second method mentioned, and which applies also to the zero-crossing method, is that control of the switching devices is slow. This means that, when AC system faults (e.g. short-circuits) occur, the switching devices used (e.g. GTOs) can be subjected to high transient overcurrents which can damage the devices.
It is an object of the present invention to provide a control system which can rapidly adapt to abnormal AC system conditions.